Rechargeable D Cell Batteries. Energizer d cell rechargeable batteries are simply a c cell in a plastic case. Will hold up to 75% of the charge after 12 months without use.
If you really need a big brand name, buy a rechargeable aa, with a d cell adapter. They also offer reliable performance with consistent power output at various currents. These rechargeable batteries come in a pack of four and can hold 5000 mah.